This invention relates to transducers employed to transform mechanical forces into an electrical signal indicative of the magnitude of an applied mechanical force or stress. Particularly, this invention relates to a method and structure enabling the fabrication of a transducer employing small and thin diaphragms while maintaining large piezoresistive coefficients in regard to the sensors deposited or otherwise employed on such diaphragms.
The use of piezoresistive transducers in conjunction with semiconductor diaphragms of all types is quite well documented in the prior art. Essentially, the art has progressed to the point where the piezoresistive transducer is a relatively reliable and economical component and is widely used to measure stress and motion in a plurality of different applications.
In any event, a major desire in the fabrication and formulation of such transducers is to achieve further economical advantages while maintaining sensitivity and reliability which are associated with such units. Essentially, an object of the prior art and of the transducer technology is to provide an economical transducer assembly which possesses good electrical characteristics and sensitivity while maintaining relatively small diaphragm size and which assembly is capable of being produced in large quantities employing conventional mass production integrated circuit techniques. Hence, there exists a number of patents in the prior art which attempt to provide a transducer structure according to the above considerations.
If reference is made to U.S. Pat. No. 3,800,264 entitled HIGH TEMPERATURE TRANSDUCERS AND HOUSINGS INCLUDING FABRICATION METHODS which issued on Mar. 26, 1974 to the Assignee herein, there is shown a dielectrically isolated transducer which employs a silicon diaphragm. The diaphragm has a piezoresistive sensor mounted on a dielectric insulator which in essence, provides isolation of the sensor from the diaphragm while further providing a high temperature bonding to a suitable housing. The structure depicted in that patent is further shown in U.S. Pat. No. 3,930,823 regarding methods of fabricating such units. Essentially, the devices depicted in the above noted patents employ small diameter diaphragms which are useful for high temperature environments and are relatively economical and reliable.
Other techniques concerned with problems of mass production and reliability are shown in U.S. Pat. No. 3,858,150 entitled POLYCRYSTALLINE SILICON PRESSURE SENSORS issued on Dec. 31, 1974. This patent describes a piezoresistive sensor which is formed in polycrystalline silicon layer. The silicon layer is deposited on an insulating etch stop layer of silicon nitride which in turn is deposited over a substrate of monocrystalline silicon. The substrate is configured to support the silicon nitride layer by etching away its inner section and leaving a peripheral section. The piezoresistive sensors are diffused into the polycrystalline layer and are located over an area that is ultimately flexible and generally designated as the active area of the diaphragm. While the technique lends itself to mass produced components, there are certain disadvantages regarding this technique. Essentially, the polycrystalline layer serves to reduce the piezoresistive coefficients of the diffused sensor elements and hence, the device employing a polycrystalline silicon layer is not as senstive as those devices which employ a homogeneous diaphragm. In polycrystalline silicon, the grains of the silicon are randomly oriented and hence the sensors which are deposited within such a layer exhibit a lower piezoresistive coefficient which is determined by the average value of the grain orientation in the crystallographic plane. It is, of course, understood and known that the gage factor of the sensor which is defined as the fractional change in resistance per unit strain, is also a function of the piezoresistive coefficient of the sensor. Hence, the higher the piezoresistive coefficient is, the more sensitive the final transducer. Furthermore, by employing a sensor with high piezoresistive coefficients, one can fabricate smaller diaphragms while maintaining reliability and optimum operating characteristics.
It is therefore an object of the present invention to provide an improved transducer structure adapted to mass production techniques while providing reliable operating characteristics by employing piezoresistive sensors having relatively high piezoresistive coefficients.